PHPMaster.com has an interesting new article posted today showing how you can use PHP to create a two-way messaging (SMS) application that can both send and initiate actions based on message content.
SMS is used for various purposes these days. For example, major websites like Gmail and Facebook use SMS to improve their authentication process with multi-factor authentication and notifying users about the updates. These are one-way SMS applications since messages are sent only from these sites to the user. Two-way SMS applications are more complex than one-way ones. In two-way SMS apps, a user can initiate a conversation by sending messages, and then the application responds according to the user's commands.
They base the application on the Clickatell SMS service (not free, but cheap - pay by the message too) which includes the ability to hook into your API on a specific endpoint and relay the message data. The message can either be sent via a POST or GET and can easily be interpreted in your app extracting things like a timestamp, the number it came from and, of course, the actual text of the message. There's also a section about the "User Data Header" functionality that lets you easily split up a message for recombination on the remote device. Code is included for all examples.